1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the zoom lens, and more particularly, to a zoom lens suitable for use in an image pickup apparatus using a solid-state image pickup element such as a video camera, an electronic still camera, a broadcast camera, or a surveillance camera, or an image pickup apparatus such as a camera using a silver-halide film.
2. Description of the Related Art
Image pickup apparatuses with increased functionality and decreased size have proliferated in recent years. An imaging optical system for use in such an image pickup apparatus generally requires a zoom lens having a compact size, a high zoom ratio, and a high resolving power.
A positive lead type zoom lens is known in which a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a rear group having at least one lens unit are arranged in order from an object side to an image side. For purposes of the following description, the side of a lens where an object to be imaged is placed is called the object side or front side of the lens; and the side of the lens where the image is formed is called the image side or back side of the lens.
In one example of a positive lead type zoom lens, first to fourth lens units respectively having positive, negative, positive, and positive refractive powers are arranged in order from the object side to the image side.
U.S. Pat. No. 6,975,461 discloses a compact zoom lens that is suitable as a telescopic zoom lens. U.S. Pat. No. 7,206,137 discloses a zoom lens having a high zoom ratio of 15 or more. U.S. Pat. No. 6,594,087 discloses a zoom lens in which five lenses respectively having positive, negative, positive, negative, and positive refractive powers are arranged in order from the object side to the image side.
To obtain a zoom lens having a high zoom ratio and a compact size, in general, the number of lenses is reduced while increasing the refractive powers of lens units that constitute the zoom lens. In such a zoom lens, however, aberration variation due to zooming increases, and errors due to tilting and eccentricity of the lenses and lens units during assembly increase.
To achieve a high optical performance of a positive lead type four- or five-unit zoom lens while ensuring a high zoom ratio and a compact lens system, it is important to appropriately set the moving parameters of the lens units for zooming.
In particular, to properly correct aberrations, such as coma aberration and curvature of field, over the entire zoom range while ensuring a compact size of the camera, it is important to appropriately set the moving parameters of the second and third lens units during zooming.